Iris

Technical Details

Cinema lenses typically use 9-15 iris blades made of hardened steel or aluminum, 0.1mm thick, for a nearly circular bokeh quality. Aperture adjustment is done via threads with 0.5-degree precision or servo motors with 1/3-stop accuracy. High-quality cine lenses achieve reproducibility of ±0.05 f-stops between identical focal lengths. The blade coating prevents internal reflections, while the aperture shape determines the characteristic bokeh and diffraction patterns at closed apertures.

History & Development

Julius Neubronner patented the first precise iris diaphragm for photographic purposes in 1902. Carl Zeiss developed the first fully automatic spring diaphragm for film cameras in 1925. Mitchell Camera Corporation introduced remote-controlled iris adjustment for studio cameras in 1935. Digital cinema cameras since 2007 enable electronic aperture ramps with programmable curves and frame-accurate synchronization to camera movements.

Practical Use in Film

Hitchcock used iris adjustment in "Vertigo" (1958) during dolly zoom shots for constant exposure with changing focal length. Kubrick worked with modified Zeiss lenses at f/0.7 in "Barry Lyndon" (1975) for candlelight scenes. Modern workflows use motorized follow-focus systems with iris control via wireless hand units or programmed lens control systems for complex camera moves with simultaneous exposure adjustment.

Comparison & Alternatives

The iris differs from ND filters by offering continuous availability without color shift, but it affects depth of field. Variable ND filters provide light reduction without depth of field change, but exhibit color casts and polarization effects. Electronic iris systems in broadcast lenses react to light fluctuations within 40ms, while mechanical irises allow for more precise manual control for narrative filmmaking. Full-frame sensors require more closed apertures (f/2.8-f/5.6) for comparable depth of field to Super35mm at f/1.4-f/2.8.